Revert "Removed -nostdlib from LDFLAGS"

This reverts commit af0cbe5.

`-nostdlib` was added to LDFLAGS when we converted to kbuild.
PR bitcraze#1310 removed -nostdlib in order to make it possible to use the `atof()` function.
However, this now causes similar linker warnings to those it was trying to solve (discussion 784).
420 days later, `atof()` is not being used anywhere internally. Discussion in bitcraze#1299 is not very conclusive on why this is needed. The arguing mostly relies that the cost in flash and RAM is so low that there is no good reason not to do it. There is now!

Makefile

@@ -37,7 +37,7 @@ LIB = $(srctree)/src/lib
 PROCESSOR = -mcpu=cortex-m4 -mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16
 LINKER_DIR = $(srctree)/tools/make/F405/linker
 
-LDFLAGS += --specs=nosys.specs --specs=nano.specs $(PROCESSOR)
+LDFLAGS += --specs=nosys.specs --specs=nano.specs $(PROCESSOR) -nostdlib
 image_LDFLAGS += -Wl,-Map=$(PROG).map,--cref,--gc-sections,--undefined=uxTopUsedPriority
 image_LDFLAGS += -L$(srctree)/tools/make/F405/linker
 image_LDFLAGS += -T $(LINKER_DIR)/FLASH_CLOAD.ld